1. Field of the Invention
The present invention relates to a semiconductor display device for displaying by a time gray scale method and a driving method thereof.
2. Description of the Related Art
As a driving method of a light emitting device that is one of semiconductor display devices, there is known a time gray scale method in which a light emission period of a pixel in one frame period is controlled with binary voltage of a digital video signal to display a gray scale. Electroluminescent materials are more suitable for a time gray scale method than liquid crystals and the like since the response speed is generally faster. Specifically, when performing display by the time gray scale method, one frame period is divided into a plurality of subframe periods. Then, a pixel emits light or does not emit light according to a video signal in each subframe period. According to the aforementioned structure, the total actual light emission period of a pixel in one frame period can be controlled by a video signal, so that a gray scale can be displayed.
However, in the case of performing display using the time gray scale method, there is a problem in that a pseudo contour may be displayed in a pixel portion depending on the frame frequency. Pseudo counters are unnatural contour lines that are often perceived when the middle gray scale is displayed by the time gray scale method, which is considered to be mainly caused by a variation of the perceptual luminance due to a characteristic of the human sight.
The pseudo contours are classified into a moving image pseudo contour which occurs when a moving image is displayed, and a still image pseudo contour which occurs when a still image is displayed. The moving image pseudo contour occurs as follow: in contiguous frame periods, a subframe period included in the previous frame period and a subframe period included in the present frame period are perceived as one continuous frame period by human eyes. That is, moving image pseudo contours correspond to unnatural bright or dark lines displayed in a pixel portion that are perceived by human eyes since the gray scale level deviates from the gray scale level to be displayed in the actual frame period. A mechanism for generation of a still image pseudo contour is the same as that of a moving image pseudo contour. The still image pseudo contour occurs when a still image is displayed, because a human viewpoint slightly moves horizontally or vertically at a boundary between regions exhibiting the different gray scale levels, and thus a moving image seems to be displayed at pixels in the vicinity of the boundary. That is, still image pseudo contours correspond to unnatural bright or dark lines that occur in a swinging manner in the vicinity of a boundary between regions exhibiting the different gray scale levels due to a moving image pseudo contour occurred at pixels in the vicinity of the boundary.
In order to prevent the above-described pseudo contours, Patent Document 1 has disclosed a driving method of a plasma display, in which a subframe period for light emission appears contiguously within one frame period. According to the driving method, such a phenomenon that a light emission period and a non-light emission period within each frame period are inverted in adjacent frame periods can be prevented, so that a pseudo contour can be suppressed.
[Patent Document 1] Japanese Patent Laid-Open No. 2000-231362 (paragraph 0023)
However, in the driving method disclosed in Patent Document 1, the total gray scale level and the number of subframe periods for one frame period are equal to each other. Therefore, when the number of subframe periods is increased in order to increase the total gray scale level, each subframe period is required to be shortened. However, video signal input to pixels at all rows is generally required in each subframe period. Thus, in the case where the subframe period is too short, the drive frequency of a driver circuit is required to be increased. When considering the reliability of a driver circuit, it is not preferable to make a subframe period shorter than is necessary.
Note that each subframe period can be lengthened to some extent by lengthening a frame period. However, lengthening the frame period is not preferable in that drastic increase of the total gray scale level is not to be realized whereas a pseudo contour is to be more generated.
In Patent Document 1, a technology for increasing the total gray scale level to be displayed in a pseudo manner without increasing the number of subframe periods is also described, in which image processing such as dithering is performed. However, by performing the image processing such as dithering, a large total gray scale level can be displayed while the image is displayed as if sand is spread thereover, leading inevitably to decrease in image quality.